practically constant. It shall be noted that directional negative sequence OC element
offers protection against all unbalance faults (phase-to-phase faults as well). Care shall
be taken that the minimum pickup of such protection function shall be set above
natural system unbalance level.
An example will be given, how sensitive-ground-fault protection for power lines can
be achieved by using negative-sequence directional overcurrent protection elements
within a CVGAPC function.
This functionality can be achieved by using one CVGAPC function. The following
shall be done to ensure proper operation of the function:
1. Connect three-phase power line currents and three-phase power line voltages to
one CVGAPC instance (for example, GF04)
2. Set
CurrentInput
to
NegSeq
(please note that CVGAPC function measures I2
current and NOT 3I2 current; this is essential for proper OC pickup level setting)
3. Set
VoltageInput
to
-NegSeq
(please note that the negative sequence voltage phasor
is intentionally inverted in order to simplify directionality
4. Set base current
IBase
value equal to the rated primary current of power line CTs
5. Set base voltage
UBase
value equal to the rated power line phase-to-phase voltage
in kV
6. Set
RCADir
to value +65 degrees (
NegSeq
current typically lags the inverted
NegSeq
voltage for this angle during the fault)
7. Set
ROADir
to value 90 degree
8. Set
LowVolt_VM
to value 2% (
NegSeq
voltage level above which the directional
element will be enabled)
9. Enable one overcurrent stage (for example, OC1)
10. By parameter
CurveType_OC1
select appropriate TOC/IDMT or definite time
delayed curve in accordance with your network protection philosophy
11. Set
PickupCurr_OC1
to value between 3-10% (typical values)
12. Set
tDef_OC1
or parameter “TD” when TOC/IDMT curves are used to insure
proper time coordination with other ground-fault protections installed in the
vicinity of this power line
13. Set
DirMode_OC1
to
Forward
14. Set
DirPrinc_OC1
to
IcosPhi&U
15. Set
ActLowVolt1_VM
to
Block
•
In order to insure proper restraining of this element for CT saturations during
three-phase faults it is possible to use current restraint feature and enable this
element to operate only when
NegSeq
current is bigger than a certain
percentage (10% is typical value) of measured
PosSeq
current in the power
line. To do this the following settings within the same function shall be done:
16. Set
EnRestrainCurr
to
On
17. Set
RestrCurrInput
to
PosSeq
18. Set
RestrCurrCoeff
to value 0.1
Section 11
1MRK 511 401-UUS A
Multipurpose protection
276
Bay control REC670 2.2 ANSI
Application manual
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